1. Field of the invention
The present invention relates to an image forming method for employing a magnetic developer attracted on the surface of a developer conveying member comprising a permanent magnetic member formed like a cylinder to develop an electrostatic latent image formed on the surface of an image-bearing member that moves while bearing the image, and in particular, to an image forming method that can prevent magnetic carriers in the magnetic developer from attaching to the image-bearing member.
2. Description of the Related Art
In the most typical conventional image forming methods in printers and facsimile terminal equipment which are applications of electrophotography or electrostatic recording, an electrostatic latent image is formed on the surface of a photosensitive drum formed like, for example, a cylinder, and a developing roller disposed opposite to the photosensitive drum and comprising a built-in permanent magnetic member and a sleeve fitted and inserted into the roller so as to share the axis with the permanent magnetic member and to rotate relative to the member is then used to deliver a magnetic developer attracted on the surface of the sleeve. Thereafter, a magnetic brush is formed in a developing region and allowed to slide on the surface on which the electrostatic latent image is formed in order to brush the surface, thereby forming a visual toner image. The developed toner image is then transferred to a transfer sheet and then thermally fixed therein.
Conventional constructions also use as charging and transfer means corona charging generated by applying a high voltage (DC 5 to 8 kV) to metal (such as stainless steel or tungsten) wires. These methods, however, also generate corona products such as ozone and NOx when generating corona, and such products may give out an offensive smell to disrupt the environment. The corona products may also degenerate the surface of the photosensitive drum to facilitate the unsharpness or degradation of images, or contaminate the wires to affect the quality of the images, resulting in the presense of undesired white sections (non-image areas) or the presence of black stripes in the images.
Since the corona transfer method electrostatically transfers a toner image to a transfer sheet by applying corona charges of a polarity opposite to that of the developer against the rear surface of the transfer sheet, the resistance of the transfer sheet may be varied due to humidity, and transfer may be difficult if the sheet has a low resistance.
In addition, since only 5 to 30% of the supplied currents reach the photosensitive drum or transfer sheet with most of the currents diverted to a shield plate, the corona discharge method has a low power efficiency as a charging or transfer means. This method thus requires a large amount of power to be consumed to obtain a predetermined efficiency and also requires a high-voltage transformer of a large capacity.
To solve the above problems, image forming methods using a roller charging means and a roller transfer means have been provided.
There have recently been strong demands for smaller devices used for the above image forming methods, and the miniaturization of a developing section is becoming more and more important. Methods that cause a magnetic developer to directly attract to the surface of a permanent magnet member and then rotate the permanent magnet member to transfer the magnetic developer without using a sleeve have thus been proposed as means for meeting such demands (for example, Japanese Patent Laid Open No. 62-201463).
FIG. 1 describes the integral part of an example of the above sleeveless developing means. In this figure, a magnetic developer 2 mainly comprises, for example, toner and magnetic carriers is accommodated in a developer vessel 1, and a permanent magnetic member 4 installed rotatably at the bottom of the developer vessel 1. The permanent magnetic member 4 has at least its surface formed so as to be conductive, and is formed like a cylinder with a plurality of axially extending magnetic poles provided on its outer circumferential surface.
The permanent magnetic member 4 can be formed of a resin bonded magnet comprising a mixture of ferromagnetic powders and resin (see Japanese Patent Laid Open No. 57-130407, Japanese Patent Laid Open No. 59-905, Japanese Patent Laid Open No. 59-226367). The surface may be formed so as to be conductive by forming a conductive layer thereon by means of bonding or plating or adding a powder-like conductive substance during the kneading of the material. The permanent magnetic member 4 may be formed of a hard ferrite magnet so as to be semi-conductive.
An image-bearing member (a photosensitive drum) 3 is rotatably installed in the direction of the arrow in FIG. 1 and opposed to the permanent magnet member 4 with a gap (g) set between the members 3 and 4. A doctor blade 5 is attached to the developer vessel 1, and opposed to the permanent magnetic member 4 with a gap (t) set between the doctor blade 5 and the member 4 to adjust the thickness of the layer of magnetic developer 2 attracted on the surface of the permanent magnet member 4. A charging roller 6, a transfer roller 7, and a cleaning device 8 having a blade 9 is disposed opposite to the outer circumference of the image-bearing member 3. In addition, a bias voltage from the permanent magnet member 4 or a DC power supply (not shown) is applied to the magnetic developer 2 attracted on the permanent magnet member 4.
With the above configuration, when the image-bearing member 3, charging roller 6, permanent magnet member 4, and transfer roller 7 are respectively rotated in the direction shown by the arrow, the charging roller 6 uniformly charges the surface of the photosensitive drum 3. When the image-bearing member 3 is then exposed with an optical signal (not shown), an electrostatic latent image is formed. The magnetic developer 2 is attracted on the permanent magnetic member 4 and is then transferred to a developing region opposite to the image-bearing member 3, where the electric field of the electrostatic latent image formed on the image-bearing member 3 causes the toner in the magnetic developer 2 to be deposited on the image-bearing member 3 , thereby developing the electrostatic latent image.
The developed toner image is transferred to the transfer sheet 10 by the transfer roller 7, moved in the direction shown by the arrow in the figure, and then fixed. After the transfer, residual toner remaining on the image-bearing member 3 is scraped away by the blade 9 that contacts the surface of the image-bearing member 3 and slides thereon, and collected in the cleaning device 8.
In this image forming method, however, magnetic carriers in the magnetic developer 2 may attach to the image-bearing member 3 together with the toner, and undesired conditions may occur if the magnetic carriers pass through the blade 9 and reach the charging roller 6. That is, since the magnetic carriers are generally conductive, leakage may occur when the magnetic carriers contact the charging roller 6 while remaining on the image-bearing member 3, thereby preventing the surface of the image-bearing member 3 from being charged uniformly, resulting in defects in the image such as noises or black spots, or even ignition of the sheet in extreme cases.
If the pressure of the blade 9 against the image-bearing member 3 is increased to completely remove the remaining magnetic toner, the surface of the image-bearing member 3 may be damaged to reduce its potential life. In addition, the disadvantage that the magnetic carriers attach to the photosensitive drum 3 is more apparent in structures with the cleaning device 8 omitted in response to the recent strong demands for the miniaturization of the apparatus.